1. Field of the Invention
This invention relates to rubber compositions having improved resistance to crack growth and cut resistance when being repeatedly subjected to relatively large strain. More particularly, it relates to rubber compositions usable for general rubber articles such as tires, rubber hoses, industrial belts, rubber vibration isolators, marine fenders, shoe soles and so on.
2. Related Art Statement
Heretofore, there have been made many studies with respect to composite bodies each consisting of rubber and resin, a greater part of which is directed to plastics having an improved shock resistance by adding a small amount of rubber to a resin, while the study with respect to rubber compositions obtained by adding a small amount of a resin to a rubber ingredient is not attempted too much as apparent from the following reasons.
In general, the plastic is used under a strain region of less than about 10%, so that even if the adhesion strength between resin and rubber is not so high, the problem of causing fracture around the interface as a fracture nucleus is relatively little.
On the other hand, the rubber composition is usually used under a strain region extending to more than 100%, but the resin blended in the rubber ingredient is hardly distorted under such a region as compared with the rubber ingredient, so that stress concentration is caused in the interface between resin and rubber to rapidly reduce the fatigue life. Therefore, the actual use of rubber composition system consisting of a rubber ingredient as a sea portion and a small amount of a resin as an island portion is an exceptional case.
However, the study of such a rubber composition has hitherto been made from a viewpoint of the enhancement of service durability. For instance, it has been confirmed from U.S. Pat. No. 4,328,133 that the resistance to crack growth and cut resistance are improved by compounding polyisoprene rubber and/or natural rubber with a small amount of short fibers as a resin. In the short fiber-reinforced rubber composition, a resin wherein a glass transition temperature of its amorphous portion is outside the ordinary temperature range (30.degree..about.120.degree. C.) is used as a material for the short fiber.
In the rubber composition of U.S. Pat. No. 4,328,133, the resistance to crack growth and cut resistance are certainly improved, but the anisotropy becomes undesirably too high. As a result, the improvement of these properties can not be attained in a certain direction. Furthermore, there is still a problem of causing stress concentration in the interface between rubber and resin to degrade the fatigue life.